1.) Field of the Invention
Mechanical stability and good corrosion resistance are important demands imposed on electrode grids for lead storage batteries. These demands on a battery ensure that the battery can be assembled without defects and has a suitable service life even under considerable stresses, e.g. the constantly rising temperatures to which starter batteries are exposed in the engine compartment of motor vehicles. Conventional batteries comprise an anode, an electrolyte; and a cathode material, separated from the anode by a separator.
However, in principle electrode grids produced using the gravity die casting process may exhibit typical casting defects, such as internal shrink holes and hot cracking. In addition to the selection of suitable alloys, the avoidance of these casting flaws is a decisive factor for the quality of the grid with regard to defect-free manufacturing and service life of the battery. Therefore, in industrial practice it is necessary to produce crack-free cast grids, and this condition is one of the factors which determine whether lead alloys can be employed in particular for starter batteries. Alloys which despite having good corrosion behavior, still cannot be formed into crack-free electrode carriers are of no practical use.
Gravity die casting is the process generally used for producing grids. The process starts with rapidly filling a grid casting die with molten lead. The molten lead is typically in the temperature range of 480-510.degree. C. The die temperature is approximately in the range from 150-200.degree. C. After the metal has been cast into the die, the heat of the melt must be dissipated into the die body through the die walls, which are coated with die coating material, until the grid has completely solidified and has cooled sufficiently for it to be possible to remove the grid.
Geometries of the electrode grids have very different cross sections in the web and frame areas. Cooling in the die takes place at a rate which varies considerably in certain locations, so that after a short time areas which have already completely solidified will be located next to areas which are still to some extent molten. Mechanical stresses produced by the non-uniform cooling and the volume contraction during solidification may therefore readily lead to the formation of heat cracks in alloys beyond a certain solidification range. The tendency to form heat cracks may be additionally promoted by the formation of low-melting phases at the grain boundaries and is in principle more of a problem for alloys that solidify in coarse grains than alloys which solidify in fine grains.
The obligation to avoid heat cracks altogether results from their effects on the electrode grids in assembling the battery, on the electrical conductivity and, in particular, on the expected service life of the positive grids, which have to withstand constant corrosion stresses.
The mechanical loads encountered in the production steps of pasting, drying and assembly may cause grids with hot cracks or even completely severed webs or frames to be destroyed or deformed in such a way that the battery may fail prematurely, for example as a result of short circuiting. The electrical conductivity of the electrode grids of both polarities determines to a large extent the output of the battery in use and may be significantly impaired in particular by crack formation in the frame and, in particular, in the vicinity of the lug. Casting defects in these areas may not be tolerated under any circumstances.
The grids in positive electrodes are exposed to constant corrosive attack as a result of the potential applied to them, and this corrosive attack, in particular in view of the high service temperatures, imposes extremely high, constantly increasing demands on the mechanical integrity and corrosion resistance of the grids. Even small casting defects are rapidly widened by the corrosion and represent a threat to the conductivity and service life of the positive electrode grids. It is therefore imperative that, in addition to highly corrosion-resistant lead alloys, only stable, crack-free grids be used in positive electrodes.
2. Description of Related Art
Lead-calcium-tin alloys are used for large numbers of electrode grids of maintenance-free lead storage batteries. Wide application ranges are known for both the calcium content and the tin content; in particular, German Patent Application 2,758,940 discloses a precipitation-hardened lead/calcium alloy in which the relative tin/calcium atomic ratio is at least 3:1 and the calcium content lies between 0.02 and 0.1% by weight. The calcium content in this known alloy is preferably approximately 0.06% by weight, and in addition, according to this publication, it has proven advantageous to add silver in quantities of between 0.02 and 0.1%, preferably approximately 0.06%, to a lead/calcium alloy.
The same type of Lead-calcium-tin alloy is also described in U.S. Pat. No. 5,298,350 and in U.S. Pat. No. 5,691,087. These documents contain further information about the advantageous effects of the addition of silver that has already been mentioned in German Patent Application 2,758,940.
Moreover, the prior art of U.S. Pat. Nos. 5,691,087 and 5,298,350 further discloses the addition of aluminum to lead/calcium alloys when casting the alloy. In this case, 0.08 to 0.012%, for example, of aluminum is added to the initial alloy composition, the quantity of aluminum added being dependant on the melting temperature during the casting process. This addition of aluminum is intended to form a passive protective layer on the surface of the molten lead and thus to reduce oxidation of the calcium or calcium burn-off.
As stated in U.S. Pat. No. 5,691,087, it is assumed that the small concentration of added aluminum does not impair the corrosion resistance of grids cast from such an alloy. According to this patent, in addition to aluminum it is also possible to use any other desired material, which is suitable as an oxygen trap in the molten material. When producing and examining grids based on the above mentioned alloys, it has been found that they did not always have sufficient corrosion resistance and reliability for practical use, because cracks which occur occasionally impair the expected performance.
In principle, the known lead-calcium-tin-silver alloys do satisfy the demands placed on corrosion resistance for use in positive electrodes, but in practice their use is only advantageous for battery quality and battery service life if it is possible to produce crack-free grids using conventional manufacturing methods.